This is an interdisciplinary program aimed at elucidating the mechanisms regulating the transport of macromolecules water and small solutes through vascular endothelium and the subendothelial tissue. A combined experimental and theoretical approach will be used. Theoretical models will guide the experimental design of the proposed research, and the experimental data will provide the essential input for the refinement of the theoretical models. The movement of tracer molecules of various sizes through the vessel wall will be studied by fluorescence and electron microscopy as a function of space and time. The results on transport will be correlated with the ultrastructure of the vessel wall and endothelial cell turnover (3Hthymidine and lgG.). The first aim is to test the hypothesis that the transient open junctions around newly replicated endothelial cells are the large pores for macromolecular transport across the endothelial cell layer in arterial vessels down to capillary level. The models and corroborating in vivo experiments will be performed on two different time scales; a short time characterizing macromolecular pasaage through the cleft and near its exit and a long time describing the diffusion is the subendothelial space. The formation of new junctions will be studied in cultured endothelial cells following a line injury. The second aim is to study the transendothelial transport of water and small solutes in relation to the ultrastructure of the normal intercellular cleft. The third aim is to study the nonisotropic convection and diffusion properties of the media as a function of wall thickness and lamellar structure. The fourth aim is to investigate the fluid filtration and macromolecular diffusion in capillary beds. The results will permit a testing of the hypotheses of a threepathway system (transient open junctions, gaps in intercellular cleft, and spaces between proteins in junctional strands as the large, intermediate and small pores, respectively) for transendothelial transport and a nonisotropic system for medial transport. The findings in these studies will provide new insights into the mechanisms regulating vascular transport in health and those leading to abnormal transport in atherogensis.